Charge (4.1.1)
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Electric current is the rate of flow of electric charge. It is measured in amperes which is one of the seven SI base units.
The electric current in passing a point can be found using the formula below:
Where:
- is the charge passing the point in coulombs , and
- is the time taken for the electric charge to pass the point in
Particles like protons and electrons carry a physical property known as electric charge, which is positive for protons and negative for electrons. Other particles, such as neutrons, do not carry any electric charge.
- Charged objects exert electrostatic forces on each other.
- Charge can be positive or negative.
- Like charges repel.
- Opposite charges attract.
Electric charge is measured in coulombs The coulomb is a derived unit. The coulomb can be converted to SI units using the current equation:
Which means that:
A value of is defined as the electric charge passing a point in a time for an electric current of :
The elementary charge is
The electric charges of ions and particles can be expressed as relative charges in terms of the elementary charge.
- The electron has a relative charge of which corresponds to an electric charge of
- The proton has a relative charge of which corresponds to an electric charge of
Ions are labelled with their relative charge. The relative charge indicates the number of elementary charges on the ion. This value is typically expressed as the number of electrons gained or lost relative to the neutral atomic form of the element.
For example, a ion has a relative charge of since it has an extra two protons compared to its number of electrons. Therefore, it has an electric charge of:
The electric charge of most objects is due to the addition or subtraction of electrons.
If electrons are added to a neutral object, the resulting net charge of the object is:
Where:
- is the elementary charge.
- The minus sign is due to the electron having a charge,
Similarly, if electrons are taken away from a neutral object, the resulting net charge of the object is:
The net charge on an object is quantised in terms of the elementary charge,
Protons, neutrons and some other exotic particles are composed of quarks, which have fractional charges in units of
However, when quarks combine to produce particles that can be observed, they always produce particles that have an integer charge in units of
Electric current is the flow of charge carriers. An electric current in a metal is due to the flow of electrons.
Metals consist of a regular lattice of positively charged metal ions with a sea of delocalised electrons, which are able to move between the ions.
Most of the electrons in a metal are localised to the atoms and do not contribute to an electric current.
Applying a voltage to a metal wire causes the delocalised electrons to move through the wire from the negative end to the positive end.
An electric current can also flow through a liquid. Electrolytes are liquids that conduct electricity. An electric current in an electrolyte is due to the flow of ions. There are two types of electrolytes:
- Ionic solutions: Formed by dissolving an ionic compound in a solvent (typically water).
- For example, salt dissolved in water is an ionic solution: separates to and ions.
- Molten ionic compounds: Formed by melting ionic compounds, allowing the positive and negative ions to carry current.
- An example of this is molten sodium chloride essentially melted table salt. When solid is heated to about it melts and its ions ( and become free to move, allowing the liquid to conduct electricity.
It is useful to note that electrolytes are essential minerals (such as sodium or magnesium). They are vital for your health because they regulate nerve and muscle function, keep you properly hydrated, and maintain your body’s fluid and pH balance.
A current can be induced through an electrolyte by placing an anode (a negatively charged electrode) and a cathode (a positively charged electrode) in an electrolyte. This produces a current in an external circuit.
For electrodes placed in an solution, the negatively charged ions are attracted to the positively charged anode, whereas the positively charged ions are attracted to the negatively charged cathode.
Each ion donates an electron to the anode, whereas each ion accepts an electron from the cathode, which results in a flow of electrons through the external circuit.
Conventional current is defined as the direction in which positive charges flow. Conventional current was defined before the discovery of the electron.
Even if the current flow is due to electrons, conventional current is still treated as flowing from a positive terminal to a negative terminal. Electron current is in the opposite direction to conventional current.
For example, the current in metals is due to electrons, which flow in the opposite direction to conventional current.
Current is measured in electric circuits using ammeters. They can be assumed to have zero resistance, meaning they have no effect on the current flow.
Ammeters are placed in series with a component to measure the current flowing through it.
The circuit symbol for an ammeter is a circle with a capital A in the centre as shown below.
The law of conservation of charge states that the net charge of an isolated system remains constant.
Kirchhoff’s first law:
- is a direct consequence of the conservation of charge and applies to electric circuits.
- states that the current flowing into a point in an electric circuit is equal to the current flowing out of the point.
Kirchhoff’s first law can be expressed mathematically:
Where:
- The symbol means ‘sum of’,
- is the curruent into the junction of an electric circuit, and
- is the curruent out of the junction.
The junction below has currents of and flowing into it:
Using Kirchhoff’s first law, this must be equal to the current flowing out of the junction, so
